1. Technical Field
The present invention relates to an objective lens drive for driving an objective lens to optically read information recorded on a rotative driven disc recording medium in a focusing direction which is parallel to an optical axis of the objective lens and in a tracking direction which is perpendicular to the optical axis.
2. Description of Related Art
As one of a supporting means for supporting an objective lens holding member in an objective lens drive which is used in an information reproduction apparatus for optically reading information recorded on a rotative driven disc recording medium, there is provided a cantilever supporting arrangement in which, for example, four metal wires are employed wherein one of the respective ends of each of the metal wires is fixed to the objective lens holding member, and each of the other respective ends thereof is fixed to a fixing part. In this cantilever supporting arrangement, a first resonance frequency of the objective lens drive is determined by the movable body mass of the objective lens drive, which includes the objective lens holding member, and mainly by the spring constants of the metal wires.
In this first resonance frequency band, the driving sensitivity to the force current supplied to the objective lens drive increases remarkably. Therefore, by designing the objective lens drive such that the first resonance frequency band generally coincides with a rotation frequency band of the recording medium, it becomes easy to ensure the acceleration required for following the displacement of the focusing direction and the tracking direction of an information recording track due to the rotation of the recording medium. Thereby, focusing the servo performance and the tracking servo performance, which are required for the reading of optical information in the information reproduction apparatus, can be ensured.
In recent years, in a CD-ROM reproduction apparatus, the speed-up of data transfer, such as four times the speed, eight times the speed, and sixteen times the speed, has been promoted against the standard transfer speed of recording information which is conventionally used in music program reproduction or the like, and these speeds have occupied the mainstream of the CD-ROM reproduction apparatus market. The speed-up of the data transfer as described above is performed by increasing the rotational speed of the disc as an information recording medium. The acceleration sensitivity, which is necessary when a light spot which is generated by the objective lens under this rotational speed is to exactly follow the displacement of the information track of the disc, is required from the objective lens drive of an optical pickup which is in charge of reading information.
In order to satisfy this requirement, the first resonance frequency is increased in accordance with an increase in the rotational speed of the disc in the conventional objective lens drive. Thereby, the conventional objective lens drive has been designed so as to obtain the maximum level of the acceleration sensitivity in the disc rotation frequency band in which the maximum displacement of the information track is generated in the focusing direction and in the tracking direction.
However, in the conventional objective lens drive, the first resonance frequency held by the objective lens drive is increased in accordance with the increase in the rotational speed of the disc, which causes a speed-up of the data transfer This results in lowering the acceleration sensitivity when the light spot generated by the objective lens follows the displacement of the information recording track in the disc rotation frequency band close to a DC band. Therefore, there has been a problem in that the operation performance under the standard data transfer speed, which is applied to music reproduction or the like as the standard function of the CD-ROM reproduction apparatus, is interfered with.
Accordingly, the present invention is made to solve these problems, and has for its object to provide an objective lens drive which can improve the light spot""s following sensitivity to the displacement of the information recording track in at least two disc rotation frequency bands.
An objective lens drive according to the present invention comprises: an objective lens holding member which holds an objective lens; a focusing coil for generating a driving force in a focusing direction that is parallel to an optical axis of the objective lens, and a tracking coil for generating a driving force in a tracking direction that is perpendicular to the optical axis of the objective lens, which are respectively wound on the peripheral part of the holding member; four first metal wires arranged so that a first end of each of the first metal wires is fixed to the objective lens holding member, and a second end of each of the first metal wires is fixed to a relay holding member; four second metal wires arranged so that a first end of each of the second metal wires is fixed to the relay holding member, and a second end of each of the second metal wires is fixed to a fixing part; and magnetic circuit members which are arranged in the fixing part so as to form the magnetic field for generating the driving force based on the Lorenzo force by supplying current to the focusing coil and the tracking coil through the first metal wires and the second metal wires, wherein the objective lens holding member is supported by the relay holding member so as to be movable in accordance with the elastic deformation of the first metal wires, and the relay holding member is supported by the fixing part so as to be movable in accordance with the elastic deformation of the second metal wires.
Accordingly, two free oscillation systems are formed. That is, a first free oscillation system is formed by the summation of the mass of the members such as the objective lens holding member, which is supported so as to be movable in accordance with the elastic deformation of the first metal wires, and spring constants of the first metal wires 61; and a second free oscillation system is formed by the summation of the mass of the members such as the objective lens holding member and the relay member, which is held on the relay holding member by four second metal wires on the fixing part and are supported so as to be movable in accordance with the elastic deformation of the second metal wires, and spring constants of four second metal wires.
The resonance phenomena which are inherent in the second free oscillation system are caused in a band lower than the resonance frequency caused by the first free oscillation system by optimizing the mass of these component members and the spring constants of the metal wires. Consequently, the acceleration sensitivity which acts on the objective lens holding member in the objective lens drive is increased in each band of the resonance frequency caused by the first free oscillation system and the resonance frequency caused by the second free oscillation system. The resonance frequency which is inherent in the first free oscillation system is generally equal to a high-speed disc rotation frequency for high-speed data transfer, thereby improving the light spot""s ability to follow the displacement of the information recording track. In addition, the light spot""s ability to follow the displacement of the information recording track in the disc rotation frequency band at the standard data transfer speed is improved by the resonance frequency generated in the second free oscillation system. Therefore, the light spot""s following sensitivity to the displacement of the information recording track can be increased in at least two disc rotation frequency bands.